Maybe it is a geforce thing. I have a p2-400, geforce(original) ddr and at 1024x768 I get 99-100fps constantly with all 7 fish. Upgrading to dx 8 had no effect on speed for me.

Oh and a DX8 tip...I heard that after you install any new version of direct X it is best to re-install both sound and vidcard drivers. I haven't needed to do so, but heard it helps some of those who have problems after the upgrade.